The use of area correlation in terminal guidance requires that the system cross-correlate a stored reference with the observed scene and have the capacity for handling variations in aspect angle, rotation, scale and intensity. This correlation must be made in real time at a low false alarm rate.
Our optical techniques can be used to perform cross-correlation and have the following advantages. An optical processor has an inherently large information capacity. A relatively modest optical system can handle scenes having over 10.sup.7 resolution elements. Such a system handles two-dimensional data in a parallel and isotropic manner with a response time dictated by the time it takes light to travel the length of the processor, plus the time required for data input and output. An increase in the number of required resolution elements does not increase the response time or size of the optical system.
Optical data processing techniques can be divided into two general categories, incoherent and coherent. Incoherent optical processing operates on the intensity of the images to be correlated, that is, it handles only positive functions. Coherent processing makes use of the phase and amplitude of the images and can therefore handle complex functions. Coherent optical correlators are well known to give distinct auto-correlation and cross-correlation peaks between data having precise scale, orientation, and contrast match. These peaks are generally quite narrow and have a low background level because correlations are performed on the high-frequency content of the input image, such as edges and other details. Correlation time is independent of the number of data points on the reference filter and the input image, although in practice the time required to obtain a correlation is determined by the data read-in time and the correlation read-out time.
In all correlation systems, variations in the input scene when compared to the on-board reference scene can cause a reduction or loss of the correlation signal. The ability of a processor to handle variations in the input scene will determine if a particular correlation technique is successful. The most common scene deviations are scale, rotational orientation, intensity, aspect angle, and overlap. A typical processor can handle errors of .+-.5% in scale. Larger errors can be handled by using additional reference images or by change in magnification of the input image. Variation in rotational orientation can be reduced by providing attitude control to the missile. A typical optical processor can handle .+-.2.degree. rotational errors. Other compensation techniques for rotational variations include using additional reference images or rotating the input optically, electronically, or digitally. A change of intensity or shading is not a problem for those systems that first obtain the Fourier transform of the scene (such as a coherent processor) for they can bandpass filter the spatial frequencies of the scene before correlation. A small change in aspect angle is a distortion of the scene and can be handled by a nonuniform magnification change across the scene area. Large aspect angle changes require that additional reference scenes be stored on board.
A sensor on board a missile will typically provide a low resolution scene for the terminal guidance system. The use of a low resolution imagery reduces the sensitivity of the system to scale and rotation errors in the input scene while still providing an adequate correlation signal (signal-to-noise ration greater than 15 dB). Additional advantages are also obtained by the use of low resolution imagery. The size of the optical elements required in the processor is reduced and the coherence requirements on the light source for the coherent optical processor are reduced, allowing laser diodes to be used. See Shareck, M. W. and Castle, J. G., Jr., Area Correlation by Fourier Transform Holography, Final Report, USA MICOM Contract DAAH01-72-C-0916, University of Alabama in Huntsville, November 1973; and Gara, A. D., "Real-Time Optical Correlation of 3-D Scenes," Appl. Opt., Vol. 16, 1977, p. 149.